Name: 2.5 Genetics Objectives At the end of this sub section students should be able to: 2.5.6 Genetic Inheritance 1. Define a gamete 2. Be familiar with gamete formation 3. Define fertilisation 4. Discuss sex determination 5. Define allele 6. Differentiate between the terms homozygous and heterozygous 7. Differentiate between genotype and phenotype 8. Differentiate between dominant and recessive 9. Understand incomplete dominance 10. Be able to complete monohybrid crosses and state the genotypes and phenotypes of parents and offspring 11. Understand the 3:1 ratio for heterozygous crosses 2.5.10.H Origin of the 12. Discuss the work of Gregor Mendel Science of genetics 2.5.11 H Law of segregation 2.5.12 H Law of independent assortment 2.5.13.H Dihybrid cross 13. State the Law of Segregation 14. Discuss the chromosomal basis for this law 15. Complete monohybrid crosses 16. State Mendel s Law of Independent Assortment 17. Discuss the chromosomal basis for this law 18. Complete dihybrid crosses 19. Know the expected ratios from Mendel s Laws 20. Define linkage 21. Identify linked genes 22. Define sex linkage 23. Give examples of sex linked characteristics 24. Discuss non nuclear inheritance Heredity is the study of how characteristics are passed on from parents to children. Gene is a section of DNA that codes for a particular characteristic e.g. hair colour. Allele - an alternative form of a gene e.g. gene for height in pea plants has two forms - tall (T) or small (t). Homozygous: The alleles of a gene pair are the same e.g. TT, tt - purebreeding. Heterozygous: The alleles of a gene pair are different (Tt) carriers. Dominant gene: A gene that is expressed in the heterozygous condition e.g. tall Tt. Recessive gene: A gene that is expressed only in the homozygous condition e.g. small tt. Genotype: The set of genes an individual possesses e.g. TT, tt or Tt. Phenotype: The physical appearance of the organism e.g. tall. It is produced from the interaction of the genotype and the environment. 07/01/2012 Page 1
A carrier of a trait has one copy of the recessive allele and therefore shows the dominant allele. Punnett square - a grid to show the possible genotypes of the offspring in a genetic cross. Male Possible gametes R r Female R RR Rr r Rr rr Autosome: a chromosome other than the sex chromosomes. Sex chromosomes: chromosomes that determine the sex of an individual - XX or XY. Monohybrid cross: a genetic cross in which only one characteristic is being examined e.g. TT x Tt Dihybrid cross: a genetic cross where two contrasting traits are investigated e.g. TtYy x TTYY. Law of segregation (Mendel s first law): A characteristic is controlled by 2 genes which separate at gamete formation e.g. Tt meiosis T or t. Law of independent assortment (Mendel s second law): when gametes are formed, each member of a pair of alleles may combine randomly with either of another pair e.g. YyRr meiosis YR or Yr or yr or yr. True-breeding (homozygous): when offspring inherit a pair of identical alleles for a trait, generation after generation. Linked genes are genes located on the same chromosome, which tend to be inherited together. Genetic code: the arrangement of genes on chromosomes. Chromosomes: Thread-like structures, that occur in the nuclei of living cells, which are made of DNA and protein and contain genes. DNA: Deoxyribonucleic acid - transmits hereditary information and controls cellular activities. Locus: The position of a gene on a chromosome ( its address ). Homologous chromosomes: Pairs of chromosomes having the same allelic genes. Multiple alleles: More than two different forms of the same gene e.g. blood grouping in man is governed by 3 alleles A, Band O. Only two of the possible number of alleles will be in any one organism. Incomplete dominance (co-dominance): this is when neither allele is totally dominant, both genes are expressed in the phenotype e.g. cattle: red x white = roan; snapdragon: red x white = pink; hens: black x white = Andalucian (slate colour). Back cross (test cross): involves the crossing of individuals of an unknown genotype (i.e. could be homozygous or heterozygous) with a known homozygous recessive. Recombination: A combination of genes present in the offspring but not present in either parent. 07/01/2012 Page 2
Sex-linkage: genes which are carried on the sex chromosomes and which determine other characteristics. They can be completely or partially sex-linked, depending o whether they are located on the X or Y section of the chromosome. Haemophilia: an inherited disease in which the blood doesn t clot properly. Albino: an organism without the pigment for the colour of hair, eyes or skin. Progeny: offspring produced Pedigree is a diagram showing the genetic history of a group of related individuals. Haploid: having a single set of unpaired chromosomes. Diploid: having chromosomes in pairs. Hybrid: an off-spring of a cross between parents. Mitosis: Produces two identical daughter cells and maintains chromsome number during cell division of somatic cells. Meiosis: Produces four daughter cells and reduces the chromsome number by half during cell division. Gamete: a haploid sex cell. Fertilisation: Mendel s experiments Explanation of Mendel s First law Mendel experimented with peas which grew in the garden of the monastery in which he lived (Brno, Czech Republic,1860s), and noticed certain differences between the plants. Some were tall, some small; some had round, smooth seeds, while others had wrinkled seeds. He selected some tall and some short plants and bred them individually until he was satisfied that he had true breeding lines, i.e. tall plants which produced only tall plants, and short plants which produced only short plants. Pea flowers usually self-pollinate. To cross two varieties he removed the stamens (before they had produced their pollen) from the flowers of one parent. When the stamens of the other parent were ripe, he brushed their pollen on to the flowers of the first parent. At the end of the summer he collected the seeds, stored them for the winter and sowed them in the following year. He transferred pollen from tall plants to flowers of short ones, and pollen of short plants to flowers of long ones. In every case he found that the next generation of plants, which he called the F 1 or first filial generation, were all tall. This suggested that the tendency towards shortness, contributed by one parent, was suppressed. As result of these observations, Mendel referred to the tallness factor as dominant and to the shortness factor as recessive. He then allowed plants of the F 1 generation to seed naturally by self-pollination, and found that he got about three times as many tall as short plants. Obviously, then, the shortness factor was still present and had been passed on to the F 2 generation. The occurrence of this as 3:1 ratio led Mendel to the conclusion that the tallness and shortness factors, carried in the F 1 generation, separate during the formation of the sex cells or gametes, so that half of the gametes carry the tallness factor and half carry the shortness factor. 07/01/2012 Page 3
Other than stem length Mendel studied seed shape (see below), seed colour (see below), flower colour (coloured or white) pod shape (inflated or constricted), pod colour (green or yellow), flower position (axial or terminal). Scientific method Mendel made a series of guesses (hypotheses) to explain the results of his experiments a pair of genes per trait, each offspring getting one gene from each parent, the genes separate at gamete formation, dominant/recessive genes, F 2 = 3:1. A hypothesis that explains all the known facts and successfully predicts many new ones is soon referred to as a theory. If a theory continues to explain and predict correctly, in more and more situations, it may eventually become a law. Law of segregation (Mendel s first law): A characteristic is controlled by 2 genes, which separate at gamete formation e.g. Tt T or t. meiosis Monohybrid cross: Parents Phenotype: Round Wrinkled Seed Shape (both homozygous) Parents Genotype: RR X rr Gametes: R r (meiosis) F 1 genotype: Phenotype: Rr Round (Fertilisation) F 1 plants (selfed) Rr X Rr (both heterozygous) Gametes R r R r F 2 Genotype: RR Rr Rr rr Phenotype: Round Round Round Wrinkled Phenotype ratio: (carrier) 3 Round: 1 Wrinkled (carrier) Question 1: In a species of plant, yellow flower colour is dominant to white. Two yellow flowers were crossed and their seeds produced 294 yellow flowers and 89 white flowers. Explain. 07/01/2012 Page 4
Dihybrid cross and the 2 nd Law: Seeds may be yellow or green in colour. They may also be round or wrinkled in shape. Let Y = yellow (dominant) y = green (recessive) R = Round (dominant) r - wrinkled (recessive) Parents Phenotype: Yellow & Round (homozygous dominant) Green & Wrinkled (homozygous recessive) Genotype: Y Y y y X R R r r Gametes: YR X yr F 1 genotype: Y R y r Phenotype: All yellow and round F 2 self-cross: Parents: Y y Y y X R r R r 07/01/2012 Page 5
Gametes: YR Yr yr yr X Yr YR yr yr F 2 generation: (by Punnett square) YR Yr yr yr YR Yr YYRR Yellow & Round yr yr Results of dihybrid cross: 9: yellow round (parental types) 3: yellow wrinkled (new combinations) 3: green round (new combinations) 1: green wrinkled (parental types) ---------------------------------------------------------------------------------- Parents: RrYy X rryy Gametes: RY Ry ry ry X ry F 1 : RrYy Rryy rryy rryy Round Round Wrinkled Wrinkled & yellow & green & yellow & green Ratio is 1:1:1:1 (If you don t get this ratio it is due to linked genes see below) ------------------------------------------------------------------------------ Parents: RRYy X rryy Gametes: 07/01/2012 Page 6
F 1 : ---------------------------------------------------------------------------------- Parents: RrYY X rryy Gametes: F 1 : Law of independent assortment (Mendel s second law): when gametes are formed, each member of a pair of alleles may combine randomly with either of another pair (if genes are not linked) e.g. YyRr YR or Yr or yr or yr. meiosis Inheritance of many human characteristics are in keep with Mendel s findings e.g. eye colour, albinism, cystic fibrosis, blood groups, polydactyly. Incomplete dominance (co-dominance): this is when neither allele is dominant, and both are expressed in the heterozygous condition e.g. cattle: red x white = roan; snapdragon/primrose: red x white = pink. Parents: Red flower X White flower RR rr Gametes: R r F 1 Rr Pink Example: If a red bull mates with a white cow, what results will be produced? If a white cow mates with a roan bull what would be the results? 07/01/2012 Page 7
Linkage Example: Drosophila fruit fly: Genes for body colour and wing length are on the one chromosome i.e. are linked. Grey body (G) and long wings (L) are dominant to black body (g) and vestigial wings (l). G with L and g with l. Parents: GGLL X ggll G G g g L L l l Gametes: GL X gl G g F 1 : L GgLl l Self-cross (if genes linked): Parents: GgLl X GgLl Gametes: GL gl GL gl F 2 : GGLL GgLl GgLl ggll 3 grey body, long wings and 1 black body and vestigial wings In the above cross all the progeny are like the parents and there are no recombinants. If the genes were not linked the expected ratio would be 9:3:3:1 when two of the F 1 flies were mated. Independent assortment does not occur between linked genes. Example: In the fruit fly, normal antennae and grey body are linked genes. The recessive alleles for both of these are also linked and produce twisted antennae and black bodies. When a grey fly with normal antennae was crossed with a black bodied fly with twisted antennae all the resultant flies were grey with normal antennae. When these F1 flies were crossed with black flies with 07/01/2012 Page 8
twisted antennae an equal number of black, twisted antennae flies and grey, normal antennae flies were produced. Can you explain how these results came about? Sex determination Autosome: a chromosome other than the sex chromosomes. Sex chromosomes: chromosomes that determine the sex of an individual - XX or XY. Parents: Mother X Father X X X Y Gametes: X (Egg) X or Y (Sperm) F 1 genotype: XX XY Phenotype: Female Male The male thus determines the sex of an offspring. Mother gives an X to everyone but father gives an X or Y chromosome. There is a 50:50 chance that any child will be male/female) Sex-linkage In man sex-linked genes (i.e. those on the X chromosome with no corresponding part on the Y chromosome) include those governing red-green colour blindness, muscular dystrophy and haemophilia (inability to clot blood). Females with both recessive genes for haemophilia do not survive beyond the first four months of gestation period. Parents: Female carrier X Male normal X H X h X H Y Gametes: X H X h X H Y F 1 X H X H X H Y X H X h X h Y Female Male Female Male Normal Normal Carrier Haemophiliac 25% chance of producing a haemophiliac child 50% chance of producing a haemophiliac son. 07/01/2012 Page 9
It is the mother that determines if the son is haemophiliac or not since the father always passes the Y chromosome to his son. ----------------------------------------------------------------------------------------------------- Colour blindness is caused by a recessive gene on the X chromosome. Parents: Female carrier X Male colour blind Genotype: Gametes: F 1 genotype: Phenotype: ------------------------------------------------------------------------------------------------------- Show the cross between a man with normal vision and a carrier woman. About 7% of males are colour blind but only 0.1% of females are. In Drosophila the gene for eye colour is carried on the X chromosome. Red eye (R) is dominant over white eye (r). Parents: Female, white eyed X Male, red eyed Genotype: Gametes: F 1 genotype: Phenotype: ------------------------------------------------------------------------------------------------------- Females need to get the two recessive genes to show the recessive trait. males show the recessive gene more often. Females can be red-green colour blind but they need to inherit the recessive gene form both their mother and father. Males are far more likely to be red-green colour-blind than females. Example: Red-green colour blindness is a sex-linked trait. If a normal man marries a woman who is a carrier of the trait, what is the likelihood that their first son would be colour blind? 07/01/2012 Page 10
---------------------------------------------------------------------------------- Non-nuclear inheritance Mitochondria and chloroplasts contain their own DNA, which indicates that they are descendants of once free-living bacteria. At human fertilisation, only the head of the sperm enters the egg. Each offspring gets a nucleus from the male parent and a nucleus plus cytoplasm from the female parent. Mitochondria are inherited from the female only. Mitochondrial DNA has been used as a molecular clock to study evolution. By measuring the amount of mutation that has happened the time that has taken for it to occur can be calculated. Damage to mitochondrial DNA may be the cause of some degenerative diseases of the brain and muscles e.g. Alzheimer s. Mitochondria generate about 90% of the energy of the cell and other cells that use a lot of energy would be dependent on them. Other examples of non-nuclear inheritance include leaf variegation in snapdragons, Parkinson s disease. *Crossing-over and separating linked genes Crossing-over is the interchange of segments between homologous chromsomes. Occurs during prophase I of meiosis. Example in Maize seeds: Coloured seeds (C) are dominant over colourless (c). Smooth kernel (S) is dominant over shrunken (s). Parents: C c X c c S s s s Gametes: C c c or S s s F 1 C c c c (if genes linked) S s s s Actual result: (After crossing-over took place) C c C c c c c c S s s s S s s s 48% 2% 2% 48% Most are like the parents due to genes being linked. A small percentage are unlike the parents due to crossing-over - these are called recombinants. Crossing-over leads to greater variation. Separation of linked genes with crossing-over can result in the same variety of gamete genotypes and offspring but their proportions will differ to that produced by independent assortment. Phenotype Ratio: Parental Phenotypes 1:1 and Recombinant Phenotypes 1:1 07/01/2012 Page 11
Multiple alleles: More than two different forms of a gene e.g. blood grouping in man is governed by 3 alleles A, B and O. Only two of the possible number of alleles will be in any one organism. Human Blood groups: 3 allelic genes A, B and O. A and B are co-dominant O is recessive. Genotype Phenotype AA A AO A BB B BO B AB AB OO O 2.5.6 Genetic Inheritance SEC Sample Paper OL 11. (9) (b) In humans an individual s sex is determined by genes located on pairs of chromosomes called sex chromosomes. The male pair is shown as XY and the female pair as XX. Show that there is an equal chance of a child being male or female by copying and completing the Punnett square in your answer book. Give the gametes of each parent and the genotypes of the offspring. (24) 2004 OL 12. (a) Explain the following terms that are used in genetics; dominance, genotype, phenotype. (9) (b) In Aberdeen Angus cattle, the polled (P) condition (absence of horns) is dominant to the horned (p) condition. A heterozygous polled bull was crossed with a horned cow. Use the following layout in your answer book to find the possible genotypes and phenotypes of the calves that may result from this cross. Heterozygous polled bull X Horned cow Genotypes of parents... Gametes...... Genotypes of calves. Phenotypes of calves. (27) 07/01/2012 Page 12
2005 OL Genetics inheritance 13. (a) For each of the following parents give the genotypes of all the possible gametes that it can produce. (i) Parent Aa. (ii) Parent AaBb. (9) 2006 OL 11. (a) Explain the following terms, which are used in genetics: allele, homozygous, genotype. (9) 2007 OL (b) (i) Name or draw the sex chromosomes that are present in a human body cell in the case of: 1. A male, 2. A female. (ii) Use a Punnet square to show that there is a fifty percent chance that fertilization will lead to a male and fifty percent chance that it will lead to a female. (27) 4. (b) In a woman the sex chromosomes are XX; in a man they are 2007 OL 11. (c) (iii) The allele for brown eye (B) is dominant to the allele for blue eye (b). Explain each of the underlined terms. (iv) Use a Punnet square to find the possible genotypes of children of parents who are both heterozygous for brown eye. State the eye colour resulting from each of these genotypes. 2008 OL 11. (b) Hair colour in humans is genetically controlled. The allele for brown hair (B) is dominant to the allele for red hair (b). (i) Explain the underlined terms. (ii) For hair colour Seán is heterozygous (Bb) and Máire is homozygous (bb). 1. What colour is Seán s hair? 2. What colour is Máire s hair? (iii) Use a Punnet square or other means to show the following: 1. the genotypes of all the gametes that Seán and Máire can produce. 2. the genotypes of the children that Seán and Máire may have. (iv) What is the probability that one of their children may have red hair? (Give your answer as a ratio or a percentage). (27) 2009 OL 11. (a) Explain the following terms as used in genetics: (i) heterozygous (ii) incomplete dominance (iii) phenotype. (9) (b) In snapdragon plants the allele for red flower (R) is incompletely dominant to the allele for white flower (r). Heterozygous plants have pink flowers. (i) Using a Punnett square, or otherwise, give the genotypes of the parents and find the genotypes and phenotypes of the offspring of the following cross: 2010 OL (ii) Pink-flowered snapdragon x Pink-flowered snapdragon If 120 new plants were produced in this cross, how many of them would you expect to have pink flowers? Explain how you got this answer. (27) 4. Choose each term from the following list and place it in Column A to match a description from Column B. The first one has been completed as an example. 07/01/2012 Page 13
Dominant Gamete Gene Mutation Genetics Genotype Column A Genetics (i) (v) Column B The study of biological inheritance The genetic make up of an individual The allele expressed in the heterozygous condition 2010 OL 11. (b) When a pure-breeding black cat was mated with a pure-breeding white cat, all the kittens were black. (i) Which fur colour, black or white, is recessive in these cats? (ii) Using capital letters for dominant and lower case letters for recessive, give: 1. The genotypes of the parent cats. 2. The genotype of the kittens. (iii) (iv) (v) (vi) Is the genotype of the kittens referred to as homozygous or heterozygous? Give a reason for your answer to part (iii). In relation to fur colour, what will be the genotypes of the gametes that these kittens will produce? What are alleles? (27) 2.5.6 Genetic Inheritance SEC Sample Paper HL 1. Answer any five of the following. (e) What term is used to describe an individual s genetic make up?.. 2005 HL 10. (b) Cystic fibrosis is a serious condition that affects the lungs and digestive system. The condition results from the inheritance of a single pair of recessive alleles. (i) Explain each of the underlined terms. (ii) Suggest why a person with a heterozygous allele pair does not suffer from the condition. (iii) If both parents are heterozygous what is the percentage chance that one of their children may inherit the condition? Explain how you obtained your answer. 2008 HL 11. (a) Explain the following terms which are used in genetics: homozygous, recessive, phenotype. (9) 2009 HL 07/01/2012 Page 14
10. (b) (i) Explain the terms alleles and dominant. 2010 HL 2. In each of the following cases read the information provided and then, from the list below, choose the correct percentage chance of obtaining the indicated offspring in each case. 0% 10% 25% 50% 75% 100% (a) (b) In the fruit fly Drosophila the allele for full wing is dominant to the allele for vestigial wing. One parent was homozygous in respect of full wing and the other parent was heterozygous. What is the % chance of obtaining offspring with full wing? % = In roses there is incomplete dominance between the allele governing red petals and the allele governing white petals. Heterozygous individuals have pink petals. A plant with pink petals was crossed with a plant with white petals. What is the % chance of obtaining offspring with white petals? % = (c) In Dalmatian dogs the allele for brown spots is recessive to the allele for black spots. The two parents were heterozygous in respect of spot colour. What is the % chance of obtaining offspring with black spots? % = 2010 HL (d) Red hair in humans is recessive to all other hair colours. A red-haired woman and a black-haired man, whose own father was red-haired, started a family. What is the % chance of obtaining offspring with red hair? % = 10. (c) Distinguish between the terms in the following pairs by writing one sentence about each member of each pair. (ii) Homozygous and heterozygous (iii) Genotype and phenotype (24) H.2.5.10 H.2.5.13 Mendelian Genetics SEC Sample Paper HL 11. (a) (i) What is meant by the term segregation as used in genetics? (ii) State the products of segregation when an organism is heterozygous in respect of the dominant allele A. (9) (b) (i) State the Law of Independent Assortment. (ii) In cattle the allele for red coat (R) is dominant to the allele for black coat (r) and the allele for straight coat (S) is dominant to the allele for curly coat (s). When a bull with a red, straight coat was mated with cows with curly, black coats it was found that calves with four different phenotypes resulted. These four phenotypes occurred in equal numbers. 1. State the genotypes of the bull and of the cows. 2. State the phenotypes of the calves. 3. Which phenotypes of the calves suggest that independent assortment has taken place? 4. Would you expect different phenotypes if the genes for coat colour and coat type were located on the same chromosome? Explain your answer. (27) 07/01/2012 Page 15
2004 HL 3. In tomato plants the allele responsible for purple stem (P) is dominant to that for green stem (p) and the allele for cut leaf (C) is dominant to the allele for potato type leaf (c). A plant with a purple stem and cut leaves was crossed with a plant with a green stem and potato type leaves. A total of 448 seeds was obtained. When the seeds were germinated four types of progeny resulted and they had the following phenotypes; 110 purple stem and cut leaves 115 green stem and potato type leaves 114 purple stem and potato type leaves 109 green stem and cut leaves What were the genotypes of the tomato plants that gave rise to these progeny? Do the progeny of this cross illustrate the Law of Independent assortment?.. Explain your answer 2005 HL 10. (c) (i) Define the following terms as used in genetics; linkage, sex linkage. (ii) Explain why linked genes do not assort independently. (iii) 2006 HL Red-green colour blindness is a sex (X)-linked condition. Normal red-green vision results from the possession of a dominant allele (C). In each of the following cases give the genotypes of the mother and of the father. 1. A family in which one daughter is red-green colour blind and one daughter has normal colour vision. 2. A family in which all the sons are red-green colour blind and all the daughters are carriers (heterozygous). (24) 12. (b) The diagram shows some of the chromosomes in the nucleus of a cell taken from a small mammal. (i) What is the sex of this individual? (ii) How many loci are marked in the diagram? (iii) A is linked to B but not to C. Is this statement correct? Explain your answer. (iv) Is D linked to d? Explain your answer. (v) What term is used to describe the allele pair Dd? (vi) Draw a diagram, similar to the one above, but in which A, B, and C are homozygous and the cell is taken from an individual of the opposite sex. (27) 07/01/2012 Page 16
2007 HL 5. (a) In genetics, what is meant by sex linkage?. (b) In humans a sex-linked recessive allele c is responsible for red-green colour blindness. Complete the blank spaces above the lines in the following cross. 2008 HL 14. (b) (vi) Name a cell organelle, apart from the nucleus, in which DNA is found. 2008 HL 11. (b) In the fruit fly, Drosophila, the allele for grey body (G) is dominant to the allele for ebony body (g) and the allele for long wings (L) is dominant to the allele for vestigial wings (l). These two pairs of alleles are located on different chromosome pairs. (i) Determine all the possible genotypes and phenotypes of the progeny of the following cross: grey body, long wings (heterozygous for both) X ebony body, vestigial wings. (ii) What is the significance of the fact that the two allele pairs are located on different chromosome pairs? (27) (c) Haemophilia in humans is governed by a sex-linked allele. The allele for normal blood clotting (N) is dominant to the allele for haemophilia (n). (i) What is meant by sex-linked? (ii) Determine the possible genotypes and phenotypes of the progeny of the following cross: haemophilic male X heterozygous normal female. (24) 2009 HL 10. (a) (i) State Mendel s Law of Segregation. (ii) Name two cell organelles, other than the nucleus, that contain DNA. ( 9) (b) In guinea pigs the allele for black hair (B) is dominant to the allele for brown hair (b) and the allele for short hair (S) is dominant to the allele for long hair (s). The alleles governing hair colour are located on a different chromosome pair to those governing hair length. 07/01/2012 Page 17
2010 HL Genetics inheritance (ii) What term is used to describe alleles that lie on the same chromosome? (iii) Why is it significant that the two pairs of alleles, mentioned above in relation to guinea pigs, are located on different chromosome pairs? (iv) Determine all the possible genotypes and phenotypes of the offspring of a cross between the following guinea pigs: Brown hair, heterozygous short hair X Heterozygous black hair, long hair 10. (c) Distinguish between the terms in the following pairs by writing one sentence about each member of each pair. (iv) Segregation and independent assortment. (24) 07/01/2012 Page 18